1. Field of the Invention
The present invention pertains to iron oxide pigments. More particularly, the present invention pertains to synthetic iron oxide pigments. Even more particularly, the present invention concerns improved methods of producing synthetic iron oxide pigments and in particular, yellow transparent iron oxide pigments and the improved pigments produced thereby.
2. Prior Art
Iron oxide pigments are produced from natural sources as well as by synthetic techniques. Generally, the naturally occurring iron oxides are both weaker and dirtier in color than their synthetic counterparts due to impurities in the oxides. Inherently, the natural pigments are opaque or semi-opaque due to their coarse particle size. The synthetic pigments minimize these disadvantages.
One class of synthetic iron oxide pigments that are coloristically unique are the relatively pure iron oxide pigments produced by solution reactions. These pigments are commonly referred to as transparent iron oxide pigments. They exhibit a high level of transparency resulting from the inherent crystalline structure of the pigment and fine crystallite size resulting from the method of manufacture. Transparent iron oxide pigments, which are extensively used in automotive metallic finishes, are generally yellow or red. Transparent yellow iron oxide pigments are produced by precipitation with an alkaline solution from water-soluble iron salts both ferric and ferrous, followed by filtration, washing and drying thereof. The transparent red iron oxide pigments are produced by calcining transparent yellow iron oxide pigments.
Transparent iron oxide pigments are ordinarily displayed in coating applications requiring high transparency while retaining an appropriate resistance to UV degradation of the organic binder upon prolonged exposure to natural sunlight. However, they are deficient in their tinctorial strength and chroma. The yellow pigment particularly suffers from lack of color strength. This makes its use in applications, such as printing ink and plastics, rather uneconomical, especially where it seeks to replace more expensive organic pigments such as azo-yellows that generally have higher color strength. Use of chrome yellow is also increasingly being objected to on environmental grounds. Thus, improvement in the color strength (or tinting strength) of yellow iron oxide would be of commercial significance.
The present invention, as will subsequently be detailed, improves upon the color strength of the yellow transparent iron oxide pigment.
Typically, transparent yellow iron oxide pigment is prepared by reacting an aqueous solution of a water-soluble iron salt such as ferrous chloride, ferrous sulfate, ferric chloride or ferric sulfate with an alkaline precipitating agent such as an alkali metal hydroxide or carbonate or an alkaline earth metal hydroxide. In U.S. Pat. No. 2,558,302, there is disclosed a process for the production of transparent yellow iron oxide pigment utilizing from a stoichiometric equivalent to an excess of strong alkali per mole of ferrous iron. Furthermore, the reference teaches the reaction being carried out at elevated temperatures and extended periods of oxidation. As above noted, the present invention modifies the referenced process to provide transparent iron oxide pigments of improved color strength.